Anesthetic Equipment and Safety Flashcards

A circle breathing system is in use with a fresh gas flow of 0.5 L/min. The soda lime canister has a grey colour indicator that has turned white. What is the MOST appropriate interpretation?

Answer: The soda lime is exhausted and must be replaced immediately

Soda lime exhaustion indicators vary by brand, but a grey-to-white colour change (as seen with some formulations such as Intersurgical Spherasorb) signifies that the CO₂-absorbing capacity is depleted. Unlike the classic purple-to-white change of ethyl violet dyes, some modern indicators use a grey baseline that becomes white when exhausted. At a low FGF of 0.5 L/min, the rebreathing fraction is high and an exhausted canister poses a serious risk of hypercapnia. Replacement is mandatory.

During low-flow anaesthesia with desflurane, the vaporiser dial is set to 6%. The measured inspired desflurane concentration is only 3.5% despite adequate fresh gas flow. Which mechanism BEST explains this discrepancy?

Answer: The vaporiser is tilted, allowing liquid desflurane to enter the bypass chamber

Desflurane vaporisers (e.g., Tec 6, D-Vapor) are electrically heated pressurised devices specifically because desflurane's SVP (88.5 kPa at 20°C) is close to atmospheric pressure, making conventional variable bypass vaporisers unreliable. If the vaporiser is tilted, liquid desflurane can flood the bypass chamber, causing erratic — and often lower — output as the liquid phase fails to vaporise consistently within the measured gas stream. The other options are less plausible: desflurane is not significantly absorbed by soda lime at clinical concentrations, and the Tec 6 design compensates for back-pressure (pumping effect).

A patient is being ventilated via a Mapleson A (Magill) circuit during spontaneous ventilation. The fresh gas flow required to prevent rebreathing is approximately equal to the patient's alveolar minute ventilation. If the same circuit is used for controlled mechanical ventilation, how does the FGF requirement change?

Answer: It increases to approximately 2–3 times the minute ventilation

The Mapleson A circuit is highly efficient for spontaneous ventilation because expiratory valve placement allows exhaled alveolar gas to be vented early in expiration before the fresh gas washes it back. During controlled ventilation, however, the expiratory valve must remain closed during inspiration, trapping exhaled gas in the circuit and causing significant rebreathing. To prevent this, an FGF of approximately 2–3 times the minute ventilation (typically >20 L/min) is required — making the Mapleson A the worst circuit for controlled ventilation. This is the inverse of the Mapleson D/E/F (Bain) circuits, which are efficient for controlled but inefficient for spontaneous ventilation.

An anaesthetic workstation alarm indicates 'hypoxic mixture'. The set O₂ flow is 2 L/min and N₂O flow is 6 L/min. The O₂ analyser reads 22%. What is the MOST likely cause?

Answer: A third gas (e.g., air or helium) is being co-administered, diluting the O₂ fraction below the alarm set-point despite an adequate O₂/N₂O ratio

Modern hypoxic guards (e.g., Link-25, ORC) maintain a minimum O₂:N₂O ratio of approximately 1:3 by volume, which corresponds to ~25% O₂ in a binary O₂/N₂O mixture. However, these mechanical interlocks only couple the O₂ and N₂O flowmeters — they do not account for a third gas diluent. If air or helium is added as a carrier gas, the total flow denominator increases, diluting the FiO₂ below the hypoxic threshold even though the O₂/N₂O interlock is functioning correctly. The analyser reading of 22% in this scenario confirms a real dilution effect. This is a recognised safety gap in traditional hypoxic guard design.

During high-frequency jet ventilation (HFJV) for airway surgery, which complication is MOST specifically related to gas trapping due to inadequate expiratory time?

Answer: Tension pneumothorax from alveolar overdistension (air-trapping auto-PEEP)

HFJV at high frequencies (typically 100–150 breaths/min) can lead to dynamic hyperinflation when the expiratory time is insufficient for complete alveolar emptying. This progressive gas trapping generates intrinsic (auto) PEEP, raising mean airway pressure and risking alveolar rupture — manifesting as pneumothorax, pneumomediastinum, or subcutaneous emphysema. This is the most immediately life-threatening complication specifically attributable to the jet mechanism. CO₂ retention can occur but is managed by flow and frequency adjustments; hypoxaemia is addressed by FiO₂; subglottic oedema is related to humidification and not the primary mechanical hazard.

A type CF (cardiac-float) applied part is required for an intracardiac catheter connected to an ECG monitor during a procedure under general anaesthesia. The maximum permissible leakage current for a type CF applied part under single-fault conditions is:

Answer: 50 µA

IEC 60601-1 classifies applied parts by their proximity to the heart and their leakage current tolerances. Type B (body) and type BF (body-float) parts have normal-condition limits of 100 µA and 10 µA respectively, rising under single-fault conditions to 500 µA and 500 µA. Type CF (cardiac-float) parts — designed for direct cardiac contact — have the most stringent limits: 10 µA under normal conditions and 50 µA under single-fault conditions. The lower threshold reflects the dramatically reduced threshold for ventricular fibrillation when current is delivered directly to the myocardium (approximately 50–100 µA), bypassing the impedance protection of skin and chest wall.